The present invention relates to optical recording/reproducing apparatus using an optical head and an LD module used for the optical recording/reproducing apparatus and the optical head.
In the current situation where various types of optical discs are available on the market, an optical head mounting two types of light sources having different wavelengths are in widespread use in order to support discs of a plurality of specifications. At the same time, a laser module (LD module) mounting two types of light sources having different wavelengths on a single chip is proposed (Japanese Patent Laid-Open No. 120568/1997). An optical head mounting such an LD module is used so that the light beam of each wavelength is individually turned on according to the type of an optical recording medium used for recording/reproducing and a recording signal of a desired medium is read.
For a photo-detection device required to read information from an optical recording medium, it is necessary to generate error signals used for tracking servo as well as a high-frequency signal (generally called an RF signal) containing recorded information. A plurality of designs, in a variety of forms, are required for a single light beam. Further, such an optical head requires means for separating an outgoing light from a light source and a reflected light from an optical recording medium from the same optical axis. A laser module used for this purpose are so designed as to comprise such split photo-detection devices and means for separating a light.
One of the tracking error detection methods is a 3-beam method according to the Japanese Patent Laid-Open No. 162457/1994. This method comprises two side beams exclusively used for tracking servo as well as a main light beam for reading recorded information on an LD module optical recording medium (of which the representative example is an optical disc). This method is characterized by a small DC offset caused by a shift of an objective lens in the radial direction of an optical disc and less subject to various groove parameters such as bit depth and size of an optical disc and ensures stable servo performance. Thus the method is one of the tracking error detection methods now in widespread use.
The aforementioned 3-beam method is disadvantageous in that it has to generate two separate side beams on top of a main beam, thus reducing the quantity of light of the main beam. Further, photo-detection devices for individually receiving side beams is also needed. This requires a correspondingly wider area reserved for photo-detection devices than in other methods. This problem presented no particular influence for an optical head according to the conventional discrete method where elements or circuits are not integrated and parts having a single function are arranged. However, in a laser module where a light source and a photo-detection device and optical splitting means are integrated onto a single element makes it difficult to reserve the area for photo-detection devices without being affected by a backlight. Moreover, in a two-wavelength laser module equipped with the aforementioned two types of light sources and independent photo-detection devices, this problem is more difficult and required solution via some method.
One method for solving the problem of the area for photo-detection devices, that is, reduction of the device mounting area, is use of the tracking methods that do not utilize side beams. The methods using main beams alone and not side means include (1) a method where an RF signal is used to generate a tracking error signal (hereinafter referred to as the TE signal), such as the phase difference detection method according to the Japanese Patent Laid-Open No. 269588/1998 and the heterodyne method; and (2) a method that uses only a main beam without using an RF signal, such as the push-pull method.
Of these method, the method under (1) cannot be applied to media that require tracking servo for an unregistered portion, such as the CD-R and DVD-R. The push-pull method splits a detector composed of a photo-detection device that receives a reflected light of a main beam spot into four devices by a split line in the track direction and a split line in the radial direction. The push-pull method obtains a tracking error signal via an arithmetic operation circuit from output of each photo-detection device and uses the TE for tracking control, since TE=0 when a spot is in the center of a track (pit) but TE greater than 0 or TE less than 0 when the spot is deflected rightward or leftward.
The push-pull method under (2) is the easiest method to obtain a tracking error signal and has been widely used since the dawn of development of the optical recording system. However, in the push-pull method, position and intensity of the spot on the detector composed of photo-detection devices may vary in case an objective lens is driven by a tracking coil and is moved in radial direction of a disc relative to another optical system, or in case a disc is inclined to an objective lens. This causes direct current variation in the generated tracking error signal. This direct current variation is called DC offset.
When tracking servo is applied with a DC offset component contained, the tracking performance is considerably degraded, in particular when a disc with large eccentricity is used, causing a possible out-of-tracking error. Thus, the push-pull method is typically used with means for removing the DC offset.
As means for removing the DC offset according to the related art, a method is known where DC offset to accompany the eccentricity of a disc is estimated in advance, and after appropriate leaning, the offset amount is corrected in performing tracking servo. Another method according to the related art is known where tracking performance in the direction of the thread of an optical head is upgraded to minimized the lens shift. Another method according to the related art is known where a mirror area (area without tracks or pits) is provided on a disc and tracking servo is applied while correcting the offset in the mirror section.
These methods require complicated signal processing, mechanism section with good response characteristics, or specially formatted discs. Thus, these methods are not often used in actual applications.
The invention, in view of the problems of the related art, aims at providing an optical head or optical recording/reproducing apparatus mounting two types of light sources, the optical head or optical recording/reproducing apparatus having a function to reduce DC offset and requiring a smaller number of photo-detection devices and smaller device mounting area than in the related art thus facilitating scale-down and an LD module used for the optical head or the optical recording/reproducing apparatus.
According to the first aspect of the invention, an optical head for optical recording/reproducing apparatus comprises: two light sources for generating laser beams having different wavelengths; detectors individually arranged for laser beams having different wavelengths; and a diffraction device for guiding a reflected light beam from an optical recording medium to a detector having a corresponding wavelength, in that the optical head uses the output obtained from the detector to perform focus error detection and tracking error detection in an optical recording medium, that reflected light beams of laser beams from two light sources from the optical recording medium are simultaneously detected via split photo-detection devices, and that when one of the two beams having different wavelengths is focused on an optical recording medium for recording/reproducing, the spot diameter of the other non-focused metering beam on the optical recording medium is set larger than that of the one of the two light beams.
In this way, by simultaneously operating two types of light sources instead of operating only one light source that matches the type of an optical recording medium used as in an optical head according to the related art, with one light beam that matches the types of the optical recording medium radiated while focused on the recording surface and the other light beam used as a non-focused beam and assigned a larger spot diameter on the recording surface than the focused light beam, the spot of the side beam is radiated in a wide range over several tracks on the disc in the radial direction. Thus, the reflected light caused by the side beam contains a negligible track cross component caused by a beam spot crossing a track (component caused by the difference in intensity between a track groove and a land).
In other words, since the spot diameter of the side beam is increased, the cut-off frequency of the optical transfer parameter (OTP) for a non-focused metering beam is shifted to the lower frequencies. This removes a higher track cross component in the space frequency (reciprocal of the track pitch) and obtains a signal containing a DC offset component alone caused by lens shift, in case the non-focused metering beam is received by photo-detection devices split and arranged via split line in the tracking direction and the difference of the outputs is obtained.
Thus, according to the invention, design is made so that the space frequency component corresponding to the track pitch maybe removed via filtering effects, by making the spot diameter of a non-focused metering beam larger than that of a main beam.
In this way, according to the invention, two types of light sources are simultaneously operated so that a light beam from one source that matches the optical recording medium used can be use for reproducing of signals and for a focus error signal. This allows shared use of photo-detection devices while DC offset is being corrected, thus reducing the number of photo-detection devices.
According to the second aspect of the invention, the spot diameter of the non-focused light beam on an optical recording medium is set to 2.5 to 15 times as large as the spot diameter of the focused light beam on the optical recording medium.
In this way, setting the spot diameter of the non-focused light beam at least 2.5 times as large as that of the main light beam allows the spot of the side beam to be radiated in a wide range over several tracks on the disc in the radial direction. Thus, the reflected light caused by the non-focused metering beam contains a negligible track cross component caused by a beam spot crossing a track (component caused by the difference in intensity between a track groove and a land). Thus a tracking error signal containing negligible DC offset can be obtained. In particular, in case the width of the non-focused metering beam in the radial direction of a disc is at least the 2.5 times as large as the spot diameter of the focused metering beam, the amplitude of the track cross component after arithmetic operation can be maintained at least 90 percent the original track cross signal.
The upper limit of the spot diameter of a non-focused metering beam is restricted because the spot diameter must be in size not overlapping with the non-focused metering beam on a detector (photo-detection device). The upper limit is further restricted by the photo detection area of the detector of the side beam. Of these restrictions, the latter is more severe. One side of the detector is typically set to about 150 micrometers or below. In a design where the 10-fold magnitude is obtained between a disc and a photo-detection device in an optical head, the spot diameter of the non-focused beam is reasonably set to 15 micrometers or below. On the other hand, the spot diameter of the non-focused beam on a disc is about one micrometer thus the spot diameter of the non-focused beam is desirably 15 times or below the spot diameter of the main beam.
According to the third aspect of the invention, the outgoing direction of a laser beam from a light source for generating two types of laser beams having different wavelengths is deflected.
In this way, by deflecting the outgoing direction of a laser beam from two types of light sources, it is possible to control the difference in the spot diameter of two types of light beams on an optical recording medium.
According to the fourth aspect of the invention, the optical recording/reproducing apparatus has an arithmetic operation circuit that obtains a tracking error signal by using signal output of both light beams from photo-detection devices.
In this way, by obtaining a tracking error signal by using signal output of both light beams from photo-detection devices, a side beam and photo-detection devices for the side beam are no longer needed to correct DC offset, thus allowing the number of photo-detection devices to be reduced.
According to the fifth aspect of the invention, the optical recording/reproducing apparatus has an arithmetic operation circuit that obtains a tracking error signal by subtracting a tracking error signal according to the push-pull method from photo-detection devices for a non-focused laser beam from an original tracking error signal according to the push-pull method from photo-detection devices for a focused laser beam.
As in the optical recording/reproducing apparatus according to the fifth aspect of the invention, by obtaining a tracking error signal by using signal output of both light beams from photo-detection devices, a side beam and photo-detection devices for the side beam are no longer needed to correct DC offset, thus allowing the number of photo-detection devices to be reduced.
According to the sixth aspect of the invention, the optical recording/reproducing apparatus has a polarity selection circuit for inverting the polarity of the output of an arithmetic operation circuit for a tracking error signal via a signal for indicating the optical recording medium currently in use.
In this way, by selecting between inversion and non-inversion of the polarity of the output of an arithmetic operation circuit for a tracking error signal, depending on which an optical recording medium is currently used, it is possible to share an arithmetic operation circuit for obtaining a tracking error signal, thus simplifying the circuit configuration.
According to the seventh aspect of the invention, the LD module is used for an optical head according to any one of the first through fourth aspects of the invention or optical recording/reproducing apparatus according to any one of the fourth through sixth aspects of the invention.
An LD module used for an optical head or optical recording/reproducing apparatus in the aforementioned configuration is advantageous in that a non-focused metering beam can be used for correcting DC offset. This reduces the number of photo-detection devices and allows scale-down of the apparatus.